Integrated circuit devices having non-volatile memory may store operating parameters of the device, operating programs and/or fixed data in the non-volatile memory. The non-volatile memory may be write protected so as to prevent unauthorized and/or inadvertent changes to the contents therein. A number of means are available to write-protect non-volatile memory and are collectively referred to herein as “write-protect fuses.” Two types of write-protect fuses may be implemented with a non-volatile memory, a) a temporary write-protect fuse that allows a user clear function (disable write-protection), and b) a permanent write-protect fuse that permanently places the non-volatile memory in a write-protected mode without the user being able to clear the permanent write-protect. Thus once a device has been programmed and permanently write-protected, the device program cannot be altered thereafter.
Typically, an integrated circuit device will have a multi-function pin (external connection) that may be utilized for either normal operation of the integrated circuit or programming operations thereto. The multi-function pin may be further characterized as requiring different voltage levels, e.g., use of a higher voltage then the operating voltage to perform a semi-permanent or permanent programming operation.
According to the JEDEC Solid State Technology Association Specification for Serial Presence Detection (SPD) electrically erasable and programmable read only memory (EEPROM), when attempting to program a temporary write-protect fuse using the SWP (software write protect) command, if a high voltage, VHV, on the A0 address pin of the device is not at a high enough voltage for completing the temporary write-protection (SWP) command, then the device will have its permanent write-protect fuse set, e.g., execute the permanent software write protect (PSWP) command instead. This results in the device being permanently write-protected, and would require the device to be physically replaced if a program change and/or correction had to be implemented thereto.
Referring to FIG. 5, depicted is a table of voltages and logic levels associated with a prior technology external multifunction input-output connection used to perform temporary write-protection (SWP) and permanent software write protect (PSWP) commands. When attempting to program a temporary write protection (fuse) using the SWP command, if the VHV level applied to the A0 input connection is insufficient, the integrated circuit device will inadvertently program the permanent write protection (fuse), i.e., execute the PSWP command instead. This renders the integrated circuit device permanently write protected.